Apparatus and method for driving a display device

ABSTRACT

An apparatus for driving a display device including a plurality of four color pixels is provided, which includes: an input unit receiving input three-color image signals; an image signal modifier converting the three-color image signals into output four-color image signals such that a maximum gray of the input three-color image signals is equal to a maximum gray of the output four-color image signals; and an output unit outputting the output four-color image signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of U.S. patentapplication Ser. No. 11/109,590 filed Apr. 18, 2005, now U.S. Pat. No.7,773,102 which claims priority to Korean Patent Application No.2004-0026751 filed on Apr. 19, 2004, the contents of which areincorporated by reference in their entireties herein.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an apparatus and a method of driving adisplay device.

(b) Description of the Related Art

Display devices include a cathode ray tube (CRT), a plasma display panel(PDP), a liquid crystal display (LCD), an organic light emitting display(OLED), etc. The display devices are used for various devices such asmonitors, television sets, indoor and outdoor signboards, etc., and theyare required to have high luminance for the television sets and thesignboards. The LCD, which is a non-emissive display, has one of maindisadvantages of low luminance.

An LCD includes two panels provided with field-generating electrodessuch as pixel electrodes and a common electrode and a liquid crystal(LC) layer with dielectric anisotropy disposed between the panels. Thepixel electrodes are arranged in a matrix and connected to switchingelements such as thin film transistors (TFTs) such that they aresupplied with data voltages row by row in a sequential manner. Thecommon electrode covers an entire surface of one of the panels and it issupplied with a common voltage. A pixel electrode and a common electrodeas well as a LC layer interposed therebetween form a LC capacitor, andthe LCD capacitor and a switching element connected thereto are thebasic elements of a pixel.

The LCD applies voltages to the field-generating electrodes to form anelectric field in the LC layer and adjusts the field strength to controltransmittance of light passing through the LC layer, thereby therealizing desired images on the display. The LCD reverses a polarity ofdata voltages, which are applied to the pixel electrodes, with respectto the common voltage by frame, row or dot in order to prevent thedeterioration of the LC layer due to long-time application of aunidirectional electric field.

In the meantime, each pixel represents a color for color display byproviding red, green or blue color filter facing the pixel electrode.

The red, greed, and blue color filters are usually arranged in stripes,in mosaics, or in deltas. The striped arrangement arranges the colorfilters such that the color filters in a column represent the samecolor, and the mosaic arrangement arranges the color filters such thatthe red, green, and blue color filters are sequentially arranged in arow direction and in a column direction. In the deltaic arrangement, thecolor filters form a plurality of rows, each row including red, green,and blue color filters arranged in sequence, and the color filters inadjacent rows are offset. Since a dot in the deltaic arrangement caninclude red, green, and blue color filters arranged in a triangle, thedeltaic arrangement has an advantage in displaying circles or obliquelines.

However, such a three color LCD has relatively low light efficiencysince the red, green, and blue color filters reduce the transmittance ofthe incident light by one thirds.

SUMMARY OF THE INVENTION

An apparatus for driving a display device including a plurality of fourcolor pixels is provided, which includes: an input unit receiving inputthree-color image signals; an image signal modifier converting thethree-color image signals into output four-color image signals such thata maximum gray of the input three-color image signals is equal to amaximum gray of the output four-color image signals; and an output unitoutputting the output four-color image signals.

The image signal modifier may: compare grays of the input three-colorimage signals, determine a maximum input gray, a middle input gray, anda minimum input gray, and assign order indices based thereon; gamma andreverse gamma converts the maximum input gray, the middle input gray,and the minimum input gray to obtain a maximum output gray (Max′), amiddle output gray (Mid′), a minimum output gray (Min′), and an outputwhite gray (W) of the output four-color image signals; and generate theoutput four-color image signals based on the order indices.

The maximum input gray (Max), the middle input gray (Mid), and theminimum input gray (Min) may have relations with the maximum output gray(Max′), the middle output gray (Mid′), the minimum output gray (Min′),and the output white gray (W) as follows:when Γ(Max)>[s ₁/(s ₁−1)]Γ(Min),Max′=Max;Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];Min′=0; andW=Max Min/Γ⁻¹[Γ(Max)−Γ(Min)],where Γ is a gamma conversion function, Γ⁻¹ is a reverse gammaconversion function, and s₁ is a scaling factor.

The maximum input gray (Max), the middle input gray (Mid), and theminimum input gray (Min) may have relations with the maximum output gray(Max′), the middle output gray (Mid′), the minimum output gray (Min′),and the output white gray (W) as follows:when Γ(Max)≦[s ₁/(s ₁−1)]Γ(Min),Max′=Max;Mid′=Γ⁻¹ {s ₁[Γ(Mid)−Γ(Max)]+Γ(Max)};Min′=Γ⁻¹ {s ₁[Γ(Min)−Γ(Max)]+Γ(Max)}; andW=Γ ⁻¹[(s ₁−1)Γ(Max)],where Γ is a gamma conversion function, Γ⁻¹ is a reverse gammaconversion function, and s₁ is a scaling factor.

The gamma function may satisfy that Γ(xy)=Γ(x)Γ(y) and Γ⁻¹(pq)=Γ⁻¹(p)Γ⁻¹(q) and it may be an exponential function. The power of the gammafunction may be equal to 2.4.

The scaling factor may be equal to two.

The gamma conversion and the reverse gamma conversion may be performedby using a look-up table.

The apparatus may further include: a gray voltage generator generating aplurality of gray voltages; and a data driver that selects gray voltagesamong the plurality of gray voltages corresponding to the outputfour-color image signals and outputs the selected gray voltages to thepixels as data voltages.

The order indices may apply an order of the grays of the inputthree-color image signals to the grays of the output four-color imagesignals.

An apparatus for driving a display device including a plurality of fourcolor pixels is provided, which includes: an input unit receiving inputthree-color image signals; an image signal modifier converting thethree-color image signals into output four-color image signals such thata gamma curve for achromatic color of the display device has noinflection point; and an output unit outputting the four-color imagesignals.

The image signal modifier may: compare grays of the input three-colorimage signals, determine a maximum input gray (Max), a middle input gray(Mid), and a minimum input gray (Min), and assign order indices basedthereon; gamma and reverse gamma converts the maximum input gray, themiddle input gray, and the minimum input gray to obtain a maximum outputgray (Max′), a middle output gray (Mid′), a minimum output gray (Min′),and an output white gray (W) of the output four-color image signals; andgenerate the output four-color image signals based on the order indices.

The maximum input gray (Max), the middle input gray (Mid), and theminimum input gray (Min) may have relations with the maximum output gray(Max′), the middle output gray (Mid′), the minimum output gray (Min′),and the output white gray (W) as follows:when Γ(Max)>[s ₁/(s ₁−1)]Γ(Min),Max′=Max;Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];Min′=0; andW=Max Min/Γ⁻¹[Γ(Max)−Γ(Min)],where Γ is a gamma conversion function, Γ⁻¹ is a reverse gammaconversion function, and s₁ is a scaling factor.

The maximum input gray (Max), the middle input gray (Mid), and theminimum input gray (Min) may have relations with the maximum output gray(Max′), the middle output gray (Mid′), the minimum output gray (Min′),and the output white gray (W) as follows:when Γ(Max)≦[s ₁/(s ₁−1)]Γ(Min),Max′=Max;Mid′=Γ⁻¹ {s ₁[Γ(Mid)−Γ(Max)]+Γ(Max)};Min′=Γ⁻¹ {s ₁[Γ(Min)−Γ(Max)]+Γ(Max)}; andW=Γ ⁻¹[(s ₁−1)Γ(max)],where Γ is a gamma conversion function, Γ⁻¹ is a reverse gammaconversion function, and s₁ is a scaling factor.

The image signal modifier may give order indices according to themaximum input gray, the middle input gray, and the minimum input graythe input three-color image signals, and generates the four-color imagesignals based on the order indices.

The gamma function may an exponential function and the scaling factormay equal to two.

A method of driving a display device including a plurality of four colorpixels is provided, which includes: assigning order indices aftercomparing grays of the input three-color image signals and determining amaximum input gray (Min), a middle input gray (Mid), and a minimum inputgray (Min); gamma converting (Γ) and reverse gamma converting (Γ⁻¹) themaximum input gray, the middle input gray, and the minimum input gray;obtaining a maximum output gray (Max′), a middle output gray (Mid′), aminimum output gray (Min′), and an output white gray (W), from relationsMax′=Max,Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)],Min′=0, andW=Max Min/Γ⁻¹[Γ(Max)−Γ(Min)],when Γ(Max)≦[s₁/(s₁−1)]Γ(Min) (where s₁ is a scaling factor); and fromrelationsMax′=Max,Mid′=Γ⁻¹ {s ₁[Γ(Mid)−Γ(Max)]+Γ(Max)},Min′=Γ⁻¹ {s ₁[Γ(Min)−Γ(Max)]+Γ(Max)}, andW=Γ ⁻¹[(s ₁−1)Γ(Max)],when Γ(Max)≦[s₁/(s₁−1)]Γ(Min); and

generating four-color image signals having the maximum output gray, themiddle output gray, the minimum output gray, and the output white grayaccording to the order given by the order indices.

The gamma conversion and the reverse gamma conversion may be performedby using a look-up table.

The method may further include: generating a plurality of gray voltages;selecting data voltages among the plurality of gray voltagescorresponding to the four-color image signals; and applying the datavoltages to the pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent by describingembodiments thereof in detail with reference to the accompanying drawingin which:

FIG. 1 is a block diagram of an LCD according to an embodiment of thepresent invention;

FIG. 2 is an equivalent circuit diagram of a pixel of an LCD accordingto an embodiment of the present invention;

FIGS. 3-5 illustrate the striped arrangements of color filters in pixelsof LCDs according to an embodiment of the present invention;

FIGS. 6-8 illustrate the mosaic pixel arrangements of color filters inLCDs according to an embodiment of the present invention;

FIG. 9 is a layout view of an exemplary TFT array panel for an LCDaccording to an embodiment of the present invention;

FIG. 10 is a sectional view of the TFT array panel shown in FIG. 9 takenalong the line X-X′;

FIG. 11A illustrates Gamut surface formed by two axes in athree-dimensional color coordinates having three axes representingluminance of three primary colors, i.e., red, green, and blue colors,respectively;

FIG. 11B illustrates decomposition of a luminance vector according to anembodiment of the present invention;

FIGS. 12A and 12B are graphs illustrating a luminance vector of a whitesignal and a luminance vector of output three-color signals according toan embodiment of the present invention;

FIG. 13 is a flow chart illustrating a method of converting thethree-color image signals into the four-color image signals according toan embodiment of the present invention;

FIG. 14 is a flow chart illustrating a method of converting thethree-color image signals into the four-color image signals according toanother embodiment of the present invention;

FIG. 15 is a graph illustrating the luminance vector of the white signaland the luminance vector of the output three-color signals according toanother embodiment of the present invention;

FIGS. 17A and 17B are graphs illustrating gamma curves that aregenerated by the methods shown in FIGS. 14 and 16;

FIGS. 18A and 18B are graphs illustrating a gamma curve of a whitesignal and a gamma curve of the output three-color signals, which aredecomposed from the gamma curve shown in FIG. 17A; and

FIGS. 19A and 19B are graphs illustrating a gamma curve of a whitesignal and a gamma curve of the output three-color signals, which aredecomposed from the gamma curve shown in FIG. 17B.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown.

In the drawings, the thickness of layers and regions are exaggerated forclarity. Like numerals refer to like elements throughout. It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” another element, it can be directly on theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly on” another element,there are no intervening elements present.

Then, apparatus and methods of driving a display device are describedwith reference to accompanying drawings.

FIG. 1 is a block diagram of a display device according to an embodimentof the present invention and FIG. 2 is an equivalent circuit diagram ofa pixel of an LCD according to an embodiment of the present invention.

Referring to FIG. 1, a display device according to an embodiment of thepresent invention includes a panel unit 300, a gate driver 400 and adata driver 500 connected to the panel unit 300, a gray voltagegenerator 800 connected to the data driver 500, and a signal controller600 controlling the above-described elements.

The panel unit 300 includes a plurality of display signal linesG₁-G_(n), and D₁-D_(m) and a plurality of pixels PX connected theretoand arranged approximately in a matrix from a circuital view as shown inFIG. 1.

The display signal lines G₁-G_(n) and D₁-D_(m) include a plurality ofgate lines G₁-G_(n) transmitting gate signals (also referred to as“scanning signals”), and a plurality of data lines D₁-D_(m) transmittingdata signals. The gate lines G₁-G_(n) extend substantially in a rowdirection and substantially parallel to each other, while the data linesD₁-D_(m) extend substantially in a column direction and substantiallyparallel to each other.

Referring to FIG. 2, each pixel PX of an LCD that is a representative ofa flat panel display includes a switching element Q connected to thesignal lines G₁-G_(n) and D₁-D_(m), and a LC capacitor C_(LC) and astorage capacitor C_(ST) that are connected to the switching element Q.The storage capacitor C_(ST) may be omitted. A pixel of an OLED (notshown) may include a switching transistor, a driving transistor, aplurality of light emitting elements, and a storage capacitor.

The switching element Q is provided on the lower panel 100 and has threeterminals: a control terminal connected to one of the gate linesG₁-G_(n); an input terminal connected to one of the data lines D₁-D_(m);and an output terminal connected to both the LC capacitor C_(LC) and thestorage capacitor C_(ST).

The LC capacitor C_(LC) includes a pixel electrode 190 provided on thelower panel 100 and a common electrode 270 provided on the upper panel200 as two terminals. The LC layer 3 disposed between the two electrodes190 and 270 functions as dielectric of the LC capacitor C_(LC). Thepixel electrode 190 is connected to the switching element Q. The commonelectrode 270 is connected to the common voltage V_(com) and coversentire surface of the upper panel 200. Unlike FIG. 2, the commonelectrode 270 may be provided on the lower panel 100, and bothelectrodes 190 and 270 have shapes of bar or stripes.

The storage capacitor C_(ST) is defined by the overlap of the pixelelectrode 190 and a separate wire (not shown) provided on the lowerpanel 100 and supplied with a predetermined voltage such as the commonvoltage V_(com). Alternatively, the storage capacitor C_(ST) is definedby the overlap of the pixel electrode 190 and its previous gate lineC_(i-1) via an insulator.

Each pixel PX represents its own color by providing one of a pluralityof color filters 230 in an area corresponding to the pixel electrode190. The color filter 230 shown in FIG. 2 is provided on the upper panel200. Alternatively, the color filters 230 are provided on or under thepixel electrode 190 on the lower panel 100.

The color of the color filter 230 is one of the primary colors such asred, green, blue, and white. Hereinafter, a pixel PX is referred to asred, greed, blue or white pixel based on the color represented by thepixel PX and indicated by reference numeral RP, GP, BP or WP, which isalso used to indicate a pixel area occupied by the pixel PX. The whitepixel WP may have no color fitter and may represent white color by meansof other mechanisms.

A pair of polarizers (not shown) polarizing incident light is attachedon the outer surfaces of the panels 100 and 200 of the panel unit 300 ofan LCD.

Spatial arrangements of the pixels of the LCDs according to embodimentsof the present invention are described with reference to FIGS. 3 to 8.

FIGS. 3-5 illustrate the striped arrangements of pixels of LCDsaccording to an embodiment of the present invention.

Referring to FIGS. 3-5, a plurality of pixels is arranged in a matrixincluding a plurality of pixel row and a plurality of pixel columns.

Each pixel row includes pixels representing four colors, i.e., redpixels RP, green pixels GP, blue pixels BP, and white pixels WP arrangedin sequence, while each pixel column includes only one kind of pixelsamong the four color pixels RP, GP, BP and WP. The sequence of thepixels in a pixel row can be altered.

A group of four pixels shown in FIGS. 3-5 form a dot, which is anelementary unit for an image.

All pixels shown in FIG. 3 have substantially equal size, while thepixels shown in FIGS. 4 and 5 do not have equal size. Referring to FIGS.4 and 5, the white pixel WP is smaller than the red, green and bluepixels RP, GP and BP to prevent the reduction of the color saturationdue to the addition of the white pixel WP. The red, green and bluepixels RP, GP and BP may have equal size.

As shown in FIG. 4, the red, green and blue pixels RP, GP and BP areenlarged and the white pixel WP is reduced, compared with those shown inFIG. 3. The ratio of the size of the white pixel WP and the size of thered, green and blue pixels RP, GP and BP is determined by consideringthe luminance of a backlight unit (not shown) and a target colortemperature. The size of the white pixel WP may be half or quarter ofother pixels RP, GP and BP.

As shown in FIG. 5, the white pixel WP is reduced while the size of thered, green and blue pixels RP, GP and BP are not changed, as comparedwith those shown in FIG. 3. The reduction of the white pixel WP isobtained by widening the signal lines such as the gate lines G₁-G_(n) orthe data lines D₁-D_(m) (shown in FIGS. 1 and 2) near the white pixel WPor by widening a portion of a black matrix (not shown), which can beprovided on the upper panel 200, enclosing the white pixel WP. It ispreferable that intersecting area between the gate lines G₁-G_(m) andthe data lines D₁-D_(m) is not increased since the intersecting areacauses capacitive load to the signal lines.

FIGS. 6-8 illustrate the mosaic pixel arrangements of LCDs according toan embodiment of the present invention.

Referring to FIGS. 6-8, each pixel row and each pixel column include twokinds of pixels among the four color pixels RP, GP, BP and WP.

Referring to FIGS. 6 and 8, the pixel rows including the green and redpixels GP and RP and the pixel rows including the blue and white pixelsBP and WP are alternately arranged. In view of columns, the pixelcolumns including the green and blue pixels GP and BP and the pixelcolumns including the red and white pixels RP and WP are alternatelyarranged.

Referring to FIG. 7, the pixel rows including the blue and red pixels BPand RP and the pixel rows including the green and white pixels GP and WPare alternately arranged. In view of columns, the pixel columnsincluding the blue and green pixels BP and GP and the pixel columnsincluding the red and white pixels RP and WP are alternately arranged.

The sequence of the pixels in a pixel row and a pixel column can be alsoaltered.

FIGS. 6-8 show a dot including a group of four pixels forming a 2×2matrix.

All pixels shown in FIG. 6 have substantially equal size, while thepixels shown in FIGS. 7 and 8 do not have equal size. Referring to FIGS.7 and 8, the white pixel WP is smaller than the red, green and bluepixels RP, GP and BP. The red, green and blue pixels RP, GP and BP mayhave equal or different sizes.

As shown in FIG. 7, the white pixel WP is reduced and the red, green andblue pixels RP, GP and BP are enlarged as compared with those shown inFIG. 6. The mosaic pixel arrangement prevents the red, green and bluepixels RP, GP and BP from being equally enlarged. As described above,the ratio of the size of the white pixel WP and the size of the red,green and blue pixels RP, GP and BP is determined by considering theluminance of a backlight unit and a target color temperature. Since thevariation of the amount of the blue light is relatively insensitive to aperson compared with red and green light, and hence, the influence ofthe areal increase of the blue pixel BP on the image quality isrelatively small, it is preferable that the increased area of the bluepixel BP is larger than those of the red pixel RP and the green pixel GPand thus the sequence of the pixels are altered as shown in FIG. 7. Thesize of the white pixel WP may be quarter of the blue pixel BP and halfof the red and green pixels RP and GP.

As shown in FIG. 8, the white pixel WP is reduced by widening both oreither of portions of the gate lines G1-Gn and the data lines D1-Dm(shown in FIGS. 1 and 2) near the white pixel WP. It is also preferablethat intersecting area between the gate lines G1-Gm and the data linesD1-Dm is not increased.

These configurations of a four color LCD increase the lighttransmittance.

Since the red, green and blue color filters transmit one thirds ofincident light, the light transmittance of a white pixel WP is aboutthree times that of other color pixels RP, GP and BP. Accordingly, theinclusion of the white pixels WP improves the optical efficiency withoutincreasing the total area of the dot.

Assume that the amount of incident light is one.

For a dot including three pixels, i.e., red, green and blue pixels, thearea of each pixel is one thirds of the total area of the dot. Since thelight transmittance of the color filter in the pixels is one thirds, thetotal light transmittance of the clot is equal to ⅓×⅓+⅓×⅓+⅓×⅓=⅓≈33.3%.

For a dot shown in FIGS. 3 and 6, the area of each pixel is a quarter ofthe total area. Since the light transmittance of the white pixel WP isone, while that of the other pixels RP, GP and BP is one thirds, thetotal light transmittance of the dot equals to ¼×⅓+¼×⅓+¼×⅓+¼×1=6/12≈50%.Accordingly, the brightness is increased to be about 1.5 times comparedwith a three-color LCD.

In addition, the reduction of the area of the white pixel WP shown inFIGS. 4, 5, 7 and 8 reduces deterioration of color level or colorsaturation (chromaticity), which may occur due to the increase of theluminance.

An exemplary detailed structure of a TFT array panel for an LCDaccording to an embodiment of the present invention will be describedwith reference to FIGS. 9 and 10.

FIG. 9 is a layout view of an exemplary TFT array panel for an LCDaccording to an embodiment of the present invention, and FIG. 10 is asectional view of the TFT array panel shown in FIG. 9 taken along theline X-X′.

A plurality of gate lines 121 are formed on an insulating substrate 110such as transparent glass.

The gate lines 121 extend substantially in a transverse direction totransmit gate signals. Each gate line 121 includes a plurality of gateelectrodes 124, a plurality of expansions 127 protruding downward; andan end portion 129 having a large area for contact with another layer ora driving circuit. The gate lines 121 may extend to be connected to adriving circuit that may be integrated on the lower panel 100.

The gate lines 121 are preferably made of Al containing metal such as Aland Al alloy, Ag containing metal such as Ag and Ag alloy, Cu containingmetal such as Cu and Cu alloy, Mo containing metal such as Mo and Moalloy, Cr, Ti or Ta. The gate lines 121 may have a multi-layeredstructure including two films having different physical characteristics.One of the two films is preferably made of low resistivity metalincluding Al containing metal, Ag containing metal, and Cu containingmetal for reducing signal delay or voltage drop in the gate lines 121.The other film is preferably made of material such as Mo containingmetal, Cr, Ta or Ti, which has good physical, chemical, and electricalcontact characteristics with other materials such as indium tin oxide(ITO) or indium zinc oxide (IZO). Good examples of the combination ofthe two films are a lower Cr film and an upper Al (alloy) film and alower Al (alloy) film and an upper Mo (alloy) film. However, they may bemade of various metals or conductors.

The lateral sides of the gate lines 121 are inclined relative to asurface of the substrate, and the inclination angle thereof ranges about30-80 degrees.

A gate insulating layer 140 preferably made of silicon nitride (SiNx) isformed on the gate lines 121.

A plurality of semiconductor islands 154 preferably made of hydrogenatedamorphous silicon (abbreviated to “a-Si”) or polysilicon are formed onthe gate insulating layer 140.

A plurality of ohmic contact islands 163 and 165 preferably made ofsilicide or n+ hydrogenated a-Si heavily doped with n type impurity areformed on the semiconductor islands 154. The ohmic contact islands 163and 165 are located in pairs on the semiconductor islands 154.

The lateral sides of the semiconductor stripes 151 and the ohmiccontacts 163 and 165 are inclined relative to a surface of thesubstrate, and the inclination angles thereof are preferably in a rangeof about 30-80 degrees.

A plurality of data lines 171, a plurality of drain electrodes 175, anda plurality of storage capacitor conductors 177 are formed on the ohmiccontacts 163 and 165 and the gate insulating layer 140.

The data lines 171 extend substantially in the longitudinal direction totransmit data voltages and intersect the gate lines 121. Each data line171 includes an end portion 179 having a large area for contact withanother layer or an external device and a plurality of source electrodes173 projecting toward the gate electrodes 124. Each pair of the sourceelectrodes 173 and the drain electrodes 175 are separated from eachother and opposite each other with respect to a gate electrode 124.

A gate electrode 124, a source electrode 173, and a drain electrode 175along with a semiconductor island 154 form a TFT having a channel formedin the semiconductor island 154 disposed between the source electrode173 and the drain electrode 175.

The storage capacitor conductors 177 overlap the expansions 127 of thegate lines 121.

The data lines 171, the drain electrodes 175, and the storage capacitorconductors 177 are preferably made of refractory metal such as Cr, Mo,Ti, Ta or alloys thereof. However, they may have a multilayeredstructure including a refractory metal film (not shown) and a lowresistivity film (not shown). Good example of the multi-layeredstructure are a double-layered structure including a lower Cr/Mo (alloy)film and an upper Al (alloy) film and a triple-layered of a lower Mo(alloy) film, an intermediate Al (alloy) film, and an upper Mo (alloy)film.

Like the gate lines 121, the data lines 171, the drain electrodes 175,and the storage conductors 177 have inclined edge profiles, and theinclination angles thereof range about 30-80 degrees.

The ohmic contacts 163 and 165 are interposed only between theunderlying semiconductor islands 154 and the overlying conductors 171and 175 thereon and reduce the contact resistance therebetween.

A passivation layer 180 is formed on the data lines 171, the drainelectrodes 175, the storage conductors 177, and the exposed portions ofthe semiconductor islands 154. The passivation layer 180 is preferablymade of inorganic insulator such as silicon nitride or silicon oxide,photosensitive organic material having a good flatness characteristic,or low dielectric insulating material that have dielectric constantlower than 4.0 such as a-Si:C:O and a-Si:O:F formed by plasma enhancedchemical vapor deposition (PECVD). The passivation layer 180 may have adouble-layered structure including a lower inorganic film and an upperorganic film so that it may take the advantage of the organic film aswell as it may protect the exposed portions of the semiconductor islands154.

The passivation layer 180 has a plurality of contact holes 182, 185 and187 exposing end portions 179 of the data lines 171, the drainelectrodes 175, and the storage conductors 177, respectively. Thepassivation layer 180 and the gate insulating layer 140 has a pluralityof contact holes 181 exposing end portions 129 of the gate lines 121.

A plurality of pixel electrodes 190 are formed on the passivation layer180, and a plurality of contact assistants 81 and 82 are formed in thecontact holes 181 and 182. The pixel electrodes 190 and the contactassistants 81 and 82 are preferably made of transparent conductor suchas ITO or IZO or reflective conductor such as Ag or Al.

The pixel electrodes 190 are physically and electrically connected tothe drain electrodes 175 through the contact holes 185 such that thepixel electrodes 190 receive the data voltages from the drain electrodes175.

Referring back to FIG. 2, the pixel electrodes 190 supplied with thedata voltages generate electric fields in cooperation with the commonelectrode 270 on the other panel 200, which determine the orientationsof liquid crystal molecules of the LC layer 3 disposed between the twoelectrodes 190 and 270 or yield currents in a light emitting layer (notshown) to emit light.

As described above, a pixel electrode 190 and a common electrode 270form a liquid crystal capacitor C_(LC), which stores applied voltagesafter the TFT forming the switching element Q is turned off. The storagecapacitors C_(ST) are implemented by overlapping the pixel electrodes190 with the gate lines 121 adjacent thereto (called “previous gatelines”). The capacitances of the storage capacitors, i.e., the storagecapacitances, are increased by providing the expansions 127 at the gatelines 121 for increasing overlapping areas and by providing the storagecapacitor conductors 177. The storage capacitor conductors 177 areconnected to the pixel electrodes 190 and overlap the expansions 127under the pixel electrodes 190 to decrease the distance between theterminals.

In one embodiment, the pixel electrodes 190 overlap the gate lines 121and the data lines 171 to increase aperture ratio. In other embodiments,the overlap is optional.

The contact assistants 81 and 82 are connected to the exposed endportions 129 of the gate lines 121 and the exposed end portions 179 ofthe data lines 171 through the contact holes 181 and 182, respectively.The contact assistants 81 and 82 protect the exposed portions 129 and179 and complement the adhesiveness of the exposed portion 129 and 179and external devices.

Referring back to FIG. 1, the gray voltage generator 800 for an LCDgenerates two sets of a plurality of gray voltages related to thetransmittance of the pixels. The gray voltages in one set have apositive polarity with respect to the common voltage Vcom, while thosein the other set have a negative polarity with respect to the commonvoltage Vcom.

The gate driver 400 is connected to the gate lines G₁-G_(n) of the panelunit 300 and synthesizes the gate-on voltage Von and the gate offvoltage Voff from an external device to generate gate signals forapplication to the gate lines G₁-G_(n).

The data driver 500 is connected to the data lines D₁-D_(m) of the panelunit 300 and applies data voltages selected from the gray voltagessupplied from the gray voltage generator 800 to the data lines D₁-D_(m).

The signal controller 600 controls the drivers 400 and 500, etc., and itincludes an image signal modifier 610. The image signal modifier 610 maybe a stand alone device.

Now, the operation of the LCD will be described in detail.

The signal controller 600 is supplied with three-color image signals R,G and B and input control signals controlling the display thereof suchas a vertical synchronization signal Vsync, a horizontal synchronizationsignal Hsync, a main clock MCLK, and a data enable signal DE, from anexternal graphic controller (not shown). The image signal modifier 610of the signal controller 610 converts the three-color image signals R, Gand B into four-color image signals and processes and modifies thefour-color image signals suitable for the operation of the panel unit300 on the basis of the input control signals and the input imagesignals R, G and B. In addition, the signal controller 600 generatesgate control signals CONT1 and data control signals CONT2 forcontrolling the processed and modified image signals Ro, Go, Bo and Wo.The signal controller 600 provides the gate control signals CONT1 forthe gate driver 400, and the processed image signals Ro, Go, Bo and Woand the data control signals CONT2 for the data driver 500.

The gate control signals CONT1 include a scanning start signal STV forinstructing to start the scanning and at least one clock signal forcontrolling the output time of the gate-on voltage Von. The gate controlsignals CONT1 may further include an output enable signal OE fordefining the duration of the gate-on voltage Von. The data controlsignals CONT2 include a horizontal synchronization start signal STH forinforming data driver 500 of the start of a horizontal period, a loadsignal LOAD or TP for instructing data driver 500 to apply theappropriate data voltages to the data lines D₁-D_(m), and a data clocksignal HCLK. The data control signals CONT2 may further include aninversion control signal RVS for reversing the polarity of the datavoltages (with respect to the common voltage Vcom).

The data driver 500 receives a packet of the image data Ro′, Go′, Bo′and Wo′ for a pixel row from the signal controller 600 and converts theimage data Ro′, Go′, Bo′ and Wo′ into the analog data voltages selectedfrom the gray voltages supplied from the gray voltage generator 800 inresponse to the data control signals CONT2 from the signal controller600. The data driver 500 then outputs the data voltages to the datalines D₁-D_(m).

Responsive to the gate control signals CONT1 supplied from the signalcontroller 600, the gate driver 400 applies the gate-on voltage V_(on)to the gate line G₁-G_(n), thereby turning on the switching elements Qconnected thereto. The data voltages applied to the data lines D₁-D_(m)are supplied to the pixels through the activated switching elements Q.

In an LCD shown in FIG. 2, the difference between the data voltage andthe common voltage Vcom applied to a pixel is expressed as a chargedvoltage of the LC capacitor C_(LC), i.e., a pixel voltage. The liquidcrystal molecules have orientations depending on the magnitude of thepixel voltage and the orientations determine the polarization of lightpassing through the LC capacitor C_(LC). The polarizers convert thelight polarization into the light transmittance.

By repeating this procedure by a unit of a horizontal period (which isindicated by 1H and equal to one period of the horizontalsynchronization signal Hsync and the data enable signal DE), all gatelines G₁-G_(n), are sequentially supplied with the gate-on voltageV_(on) during a frame, thereby applying the data voltages to all pixels.When the next frame starts after finishing one frame, the inversioncontrol signal RVS applied to the data driver 500 is controlled suchthat the polarity of the data voltages is reversed (which is called“frame inversion”). The inversion control signal RVS may be alsocontrolled such that the polarity of the data voltages flowing in a dataline in one frame are reversed (which is called “line inversion”), orthe polarity of the data voltages in one packet are reversed (which iscalled “dot inversion”).

Now, a method of converting three-color image signals into four-colorimage signals is described in detail.

First, a rule for converting the three-color image signals into thefour-color image signals according to an embodiment of the presentinvention is described in detail with reference to FIGS. 11A and 11B.

FIG. 11A illustrates Gamut surface formed by two axes in athree-dimensional color coordinates having three axes representingluminance of three primary colors, i.e., red, green, and blue colors,respectively, and FIG. 11B illustrates decomposition of a luminancevector according to an embodiment of the present invention.

A set of image signals, i.e., red, green, and blue image signals arerepresented by grays that in turn represent luminance. A gamma curve isa curve indicating a relation between gray and luminance. In the presentdescription, the terms “gray” and “grays” refer to gray levelsrepresenting the image signals. A gamma conversion is a mapping from thegray into the luminance and a reverse gamma conversion is a reversemapping of the gamma conversion, i.e., a mapping from the luminance tothe gray. The gray is denoted by GV, the luminance is denoted by L, anda gamma function representing the gamma curve is denoted by Γ. Then,L=Γ(GV); andGV=Γ ⁻¹(L).  Relation 1

If the gamma function is an exponential function,L=α(GV)^(γ); andGV=βL ^(1/γ)  Relation 2Here, α, β and γ are constants, in particular, γ is referred to as gammaconstant, and β=α^(−1/γ).

When the gamma function is an exponential function, etc., the gammafunction satisfies,Γ(xy)=Γ(x)Γ(y); andΓ⁻¹(pq)=Γ(p)Γ⁻¹(q).  Relation 3

In addition, the gamma function is an increasing function of the grayand a one-to-one function.

In the meantime, a set of three-color image signals are indicated by apoint in a gray space that have three axes representing grays ofrespective colors and by a point in a luminance space that have threeaxes representing the luminance of respective colors as shown in FIG.11A.

Since a point in a three-dimensional space can be represented by avector, a point in a gray space is referred to as gray vector and apoint in a luminance space is referred to as luminance vector. Then, fora set of three-color image signals, the relation between a gray vector{right arrow over (GV)}=(GV₁, GV₂, GV₃) and a luminance vector {rightarrow over (L)}=(L₁, L₂, L₃) is given by:{right arrow over (L)}=(Γ(GV ₁),Γ(GV ₂),Γ(GV ₃))≡{right arrow over(Γ)}({right arrow over (GV)}); and{right arrow over (GV)}=(Γ⁻¹(L ₁),Γ⁻¹(L ₂),Γ⁻¹(L ₃))≡{right arrow over(Γ)}⁻¹({right arrow over (L)})  Relation 4

If different gamma curves are given to different colors, respectivecomponents of the vector relation have different gamma functions.

Referring to FIG. 11A, a square area (or a three-dimensional cubic area)enclosed by solid lines is an area that can be represented as the inputthree-color image signals, and a hexagonal area by solid lines is anarea that can be represented as the output four-color image signals. Thehexagonal area is obtained by extending the square area along adiagonal. Therefore, the conversion of the three-color image signalsinto the four-color image signals is equivalent to a mapping of a pointin the square into a point in the hexagon.

This will be described more in detail.

A gray vector for a set of input three-color image signals is denoted by{right arrow over (I)}=(I₁, I₂, I₃), where I₁, I₂, and I₃ are grays ofthe input three-color image signals. The gray vector is gamma convertedas follows:

$\begin{matrix}{{\overset{\rightarrow}{I} = {{\begin{pmatrix}I_{1} \\I_{2} \\I_{3}\end{pmatrix}\overset{r}{\longrightarrow}{\overset{\rightarrow}{L}}_{i}} = {{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)} = \begin{pmatrix}{\Gamma\left( I_{1} \right)} \\{\Gamma\left( I_{2} \right)} \\{\Gamma\left( I_{3} \right)}\end{pmatrix}}}},} & {{Relation}\mspace{14mu} 5}\end{matrix}$where {right arrow over (L)}_(i) is a luminance vector for the inputthree-color image signals.

Next, the luminance vector is multiplied by a scaling factor thatreflects the luminance increase caused by adding a white pixel. Thescaling factor is determined by characteristics of a display device anda gamma curve. The multiplication of the scaling factor corresponds tothe above-describe mapping of a point in the square into a point in thehexagon.

$\begin{matrix}{{{\overset{\rightarrow}{L}}_{i} = {{{\begin{pmatrix}{\Gamma\left( I_{1} \right)} \\{\Gamma\left( I_{2} \right)} \\{\Gamma\left( I_{3} \right)}\end{pmatrix}\overset{s}{\longrightarrow}s}{\overset{\rightarrow}{L}}_{i}} = {{s\;{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)}} = \begin{pmatrix}{s\;{\Gamma\left( I_{1} \right)}} \\{s\;{\Gamma\left( I_{2} \right)}} \\{s\;{\Gamma\left( I_{3} \right)}}\end{pmatrix}}}},} & {{Relation}\mspace{14mu} 6}\end{matrix}$where s is the scaling factor and s{right arrow over (L)}_(i) isreferred to as an magnified vector.

The scaling factor has different values in different areas in theluminance space. For example, it is assumed that X and Y represent thecolors of the image signals having the minimum luminance and the maximumluminance, respectively, or vice versa in FIG. 11.

When the input three-color signals belong to a triangular area adjacentto the axes, the scaling factor has a value that varies depending on theluminance of the input three-color image signals and the triangular areais referred to as “variable scaling area” (VS). When the inputthree-color signals belong to a triangular area adjacent to a diagonalC, the scaling factor has a fixed value and the triangular area isreferred to as “fixed scaling area” (FS).

For example, let the scaling factor for the fixed scaling area FS equalto a constant s₁ and equal to s₂ for the variable scaling area VS, whichis given by

$\begin{matrix}{{s_{2} = \frac{\Gamma\left( \max \right)}{{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}}},} & {{Relation}\mspace{14mu} 7}\end{matrix}$where max and min denote maximum and minimum values of the three-colorimage signals.

In FIG. 11A, a boundary surface BP₁ between the fixed scaling area FSand the variable scaling area VS are written by:

$\begin{matrix}{Y = {{\frac{s_{1}{\Gamma\left( {GV}_{\max} \right)}}{{s_{1}{\Gamma\left( {GV}_{\max} \right)}} - {\Gamma\left( {GV}_{\max} \right)}}X} = {\frac{s_{1}}{s_{1} - 1}{X.}}}} & {{Relation}\mspace{14mu} 8}\end{matrix}$

Another boundary surface BP₂ between the fixed scaling area FS and thevariable scaling area VS are written by:

$\begin{matrix}{Y = {\frac{s_{1}}{s_{1} - 1}{X.}}} & {{Relation}\mspace{14mu} 9}\end{matrix}$

Here, GV_(max) is the highest gray value for each image signal.

If Y axis represent the maximum image signal, the luminance vector ofthe input three-color image signals is disposed over a diagonal plane Crepresented by X=Y. On the contrary, the luminance vector is disposedbelow the plane C if X axis represent the maximum image signal.

Accordingly, if

$\begin{matrix}{{{\Gamma\left( \max \right)} > {\frac{s_{1}}{s_{1} - 1}{\Gamma\left( \min \right)}}},} & {{Relation}\mspace{14mu} 10}\end{matrix}$the image signals belong to the variable scaling area VS. If not, theimage signals belong to the fixed scaling area FS.

In particular, when a white subpixel is as large as red, green, and bluesubpixels, the luminance is increased twice. In the above-describedstripe arrangement, for example, when incident light amount is one, thetotal transmittance is about 33.3% since the size of each subpixel andthe transmittance of each color filter is about one thirds. Since thetransmittance of a white subpixel is one, the white subpixel having thesame size as the other subpixels exhibits the transmittance of 33.3% andthus the total transmittance is increased twice. Therefore, s₁=2 isreasonable.

The luminance vector s{right arrow over (L)}_(i) obtained from Relation6 is expressed as the addition of two vectors, a luminance vector {rightarrow over (L)}_(o) covered by red, green, and blue subpixels and aluminance vector {right arrow over (L)}_(w) covered by a white subpixelas shown in FIG. 11B.s{right arrow over (L)} _(i) ={right arrow over (L)} _(o) +{right arrowover (L)} _(w).  Relation 11

Since white light is obtained by composing red, green, and blue light inequal ratio,

$\begin{matrix}{{{\overset{\rightarrow}{L}}_{w} = {{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{W} \right)} = \begin{pmatrix}{\Gamma(W)} \\{\Gamma(W)} \\{\Gamma(W)}\end{pmatrix}}},} & {{Relation}\mspace{14mu} 12}\end{matrix}$where {right arrow over (W)} is a gray vector for a white image signal.

Since the luminance vector {right arrow over (L)}_(w), of the whitesignal is given by Relation 12, the luminance vector {right arrow over(L)}_(o) of remaining three-color image signals is given by:

$\begin{matrix}\begin{matrix}{{\overset{\rightarrow}{L}}_{o} = {{s{\overset{\rightarrow}{L}}_{i}} - {\overset{\rightarrow}{L}}_{w}}} \\{= {{s\;{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)}} - {\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{W} \right)}}} \\{= {\begin{pmatrix}{{s\;{\Gamma\left( I_{1} \right)}} - {\Gamma(W)}} \\{{s\;{\Gamma\left( I_{2} \right)}} - {\Gamma(W)}} \\{{s\;{\Gamma\left( I_{3} \right)}} - {\Gamma(W)}}\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 13}\end{matrix}$

Next, the luminance vector {right arrow over (L)}_(o) of the outputthree-color image signals is reverse gamma converted to generate a grayvector {right arrow over (O)}.

$\begin{matrix}\begin{matrix}{{\overset{\rightarrow}{L}\overset{r^{- 1}}{\longrightarrow}\overset{\rightarrow}{O}} = {{\overset{\rightarrow}{\Gamma}}^{- 1}\left( {\overset{\rightarrow}{L}}_{o} \right)}} \\{= {{\overset{\rightarrow}{\Gamma}}^{- 1}\left( {{s\overset{\rightarrow}{\;\Gamma}\left( \overset{\rightarrow}{I} \right)} - {\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{W} \right)}} \right)}} \\{= \begin{pmatrix}{\Gamma^{- 1}\left( {{s\;{\Gamma\left( I_{1} \right)}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s\;{\Gamma\left( I_{2} \right)}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s\;{\Gamma\left( I_{3} \right)}} - {\Gamma(W)}} \right)}\end{pmatrix}} \\{= {\begin{pmatrix}O_{1} \\O_{2} \\O_{3}\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 14}\end{matrix}$

Then, a method of modifying image signals based on this basic ruleaccording to an embodiment of the present invention is described indetail with reference to FIGS. 12A, 12B and 13.

FIGS. 12A and 12B are graphs illustrating a luminance vector of a whitesignal and a luminance vector of output three-color signals according toan embodiment of the present invention.

An example shown in FIGS. 12A and 12B classifies the conversion in thefixed scaling area FS into two cases.

First, referring to FIG. 12A, each component of the luminance vector{right arrow over (L)}_(w) of the white signal is determined as theminimum of the components of the magnified vector s₁{right arrow over(L)}_(i). Since the minimum of the components of the magnified vectors₁{right arrow over (L)}_(i) is equal to s₁Γ(min), Relation 15 isderived from Relation 12:

$\begin{matrix}{{\overset{\rightarrow}{L}}_{w} = {\begin{pmatrix}{s_{1}{\Gamma\left( \min \right)}} \\{s_{1}{\Gamma\left( \min \right)}} \\{s_{1}{\Gamma\left( \min \right)}}\end{pmatrix}.}} & {{Relation}\mspace{14mu} 15}\end{matrix}$The gray of white image signal is given by:W=Γ ⁻¹(s ₁Γ(min)).  Relation 16

However, the gray W of the white image signal obtained by Relation 16should not be higher than the highest gray W_(max). Therefore, when thegray W of the output white image signal calculated from Relation 16 ishigher than the highest gray or W_(max) or s₁Γ(min) is larger than theluminance Γ(W_(max)) of the highest gray, that is,W>W _(max), or  Relation 17s ₁Γ(min)>Γ(W _(max)),  Relation 18a rule shown in FIG. 12B is employed to determine the gray of outputimage signals.

Referring to FIG. 12B, the luminance vector {right arrow over(L)}_(w,max) of the white image signal is defined as:

$\begin{matrix}{{\overset{\rightarrow}{L}}_{w,\max} = {\begin{pmatrix}{\Gamma\left( \max \right)} \\{\Gamma\left( \max \right)} \\{\Gamma\left( \max \right)}\end{pmatrix}.}} & {{Relation}\mspace{14mu} 19}\end{matrix}$

Accordingly, the luminance vector {right arrow over (L)}_(o) of theoutput three-color image signals is calculated from:{right arrow over (L)} _(o) =s{right arrow over (L)} ₁ −{right arrowover (L)} _(w,max).  Relation 20

Now, a method of converting the three-color image signals into thefour-color image signals based on the rule described with reference toFIGS. 12A and 12B according to an embodiment of the present invention isdescribed in detail with reference to FIGS. 13 and 14.

FIG. 13 is a flow chart illustrating a method of converting thethree-color image signals into the four-color image signals according toan embodiment of the present invention.

When a set of red, green, and blue signals having gray values R_(i),G_(i), and B_(i) are received (S10), the input image signals are gammaconverted to obtain luminance values Lr_(i), Lg_(i), and Lb_(i) (SE).

Next, the maximum M₁ and the minimum M₂ among the luminance values aredetermined (S12). That is,M ₁=Max(Lri _(i) ,Lr _(i) ,Lr _(i)), and  Relation 21M ₂=Min(Lri _(i) ,Lr _(i) ,Lr _(i)).  Relation 22

Here, Max(x, y, . . . ) means the maximum among x, y, . . . and Min(x,y, . . . ) means the minimum among x, y, . . . .

Since the gamma function is an increasing function as described above,Max(Lr_(i), Lg_(i), Lb_(i))=Γ(max) and Min(Lr_(i), Lg_(i),Lb_(i))=Γ(min), where max and min are the maximum and the maximum amongthe gray values R_(i), G_(i), and B_(i) of the input image signals,respectively.

Next, it is determined whether

$\begin{matrix}{{{M_{1} - {\frac{s_{1}}{s_{1} - 1}M_{2}}} > 0},} & {{Relation}\mspace{14mu} 23}\end{matrix}$and it is determined which area the set of the image signals R_(i),G_(i), and B_(i) belongs to among the variable scaling area VS and thefixed scaling area FS (S13). Relation 23 and Relation 10 indicatesubstantially the same relation and s₁ is a scaling factor in the fixedscaling area FS as described above.

If the image signal set satisfies Relation 23, the set of image signalsis determined to belong to the variable scaling area VS, and thus thescaling factor s is given from Relation 7 as follows (S14):s=M ₁/(M ₁ −M ₂).  Relation 24

Otherwise, i.e., if the input image signal set does not meet Relation23, it is considered to belong to the fixed scaling area FS and thus thescaling factor s is given as follows (S15):s=s ₁.  Relation 25

Next, the scaling factor s is multiplied to the luminance values Lr_(i),Lg_(i), and Lb_(i) to calculate magnification values Lr, Lg, and Lb(S16).

$\begin{matrix}{\begin{pmatrix}{Lr} \\{Lg} \\{Lb}\end{pmatrix} = {\begin{pmatrix}{sLr}_{i} \\{sLg}_{i} \\{sLg}_{i}\end{pmatrix}.}} & {{Relation}\mspace{14mu} 26}\end{matrix}$

Thereafter, preliminary luminance value L′w of the white signal isextracted (S17). The preliminary luminance value L′w of the white signalis determined as the minimum of the magnification values Lr, Lg, and Lb.That is,L′w=Min(Lr,Lg,Lb).  Relation 27

Next, it is determined whether the luminance value L′w of the extractedwhite signal is higher than the maximum luminance L_(max)=Γ(W_(max))that can be represented by the white subpixel, for example, the 255-thgray among the zero-th to the 255-th grays (S18). That is, it isdetermined whetherL′w=L _(max).  Relation 28

If the luminance value L′w satisfies Relation 28, the luminance Lw ofthe white signal is determined by (S19):Lw=L _(max)=Γ(W _(max)).  Relation 29

However, it Relation 29 is not satisfied, the luminance Lw of the whitesignal is determined to be equal to the value calculated from Relation27. That is,Lw=L′w=Min(Lr,Lg,Lb).  Relation 30

Next, the luminance values Lr_(o), Lg_(o), and Lb_(o) of red, green, andblue output image signals are determined to be equal to themagnification values Lr, Lg, and Lb subtracted by the luminance valuesLw of the white signal (S20). That is,Lr _(o) =Lr−Lw;Lg _(o) =Lg−Lw; andLb _(o) =Lb−Lw.  Relation 31

The luminance values Lw, Lr_(o), Lg_(o), and Lb_(o) of white, red,green, and blue output signals are reverse gamma inverted to obtain thegray values Wo, Ro, Go, and Bo of the four-color image signals (S21).

In the meantime, maximum, middle, and, minimum among the gray values ofthe input three-color image signals are denoted as max, mid, and min.The gray vector {right arrow over (I)} of the input three-color imagesignals are considered to have max, mid, and min as its components.

$\begin{matrix}{\overset{\rightarrow}{I} = {\begin{pmatrix}\max \\{mid} \\\min\end{pmatrix}.}} & {{Relation}\mspace{14mu} 32}\end{matrix}$

Therefore, the luminance vector of the input three-color image signals{right arrow over (L)}_(i), the magnified vector s{right arrow over(L)}_(i), the luminance vector {right arrow over (L)}_(o) of the outputthree-color image signals, and the gray vector {right arrow over (O)}are given by:

$\begin{matrix}{{{\overset{\rightarrow}{L}}_{i} = {{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)} = \begin{pmatrix}{\Gamma\left( \max \right)} \\{\Gamma({mid})} \\{\Gamma\left( \min \right)}\end{pmatrix}}};} & {{Relation}\mspace{14mu} 33} \\{{{s{\overset{\rightarrow}{L}}_{i}} = {{s\;{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)}} = \begin{pmatrix}{s\;{\Gamma\left( \max \right)}} \\{s\;{\Gamma({mid})}} \\{s\;{\Gamma\left( \min \right)}}\end{pmatrix}}};} & {{Relation}\mspace{14mu} 34} \\\begin{matrix}{{\overset{\rightarrow}{L}}_{o} = {{s{\overset{\rightarrow}{L}}_{i}} - {\overset{\rightarrow}{L}}_{w}}} \\{= {{s\;{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)}} - {\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{W} \right)}}} \\{{= \begin{pmatrix}{{s\;{\Gamma\left( \max \right)}} - {\Gamma(W)}} \\{{s\;{\Gamma({mid})}} - {\Gamma(W)}} \\{{s\;{\Gamma\left( \min \right)}} - {\Gamma(W)}}\end{pmatrix}};\mspace{14mu}{and}}\end{matrix} & {{Relation}\mspace{14mu} 35} \\\begin{matrix}{\overset{\rightarrow}{O} = {{\overset{\rightarrow}{\Gamma}}^{- 1}\left( {\overset{\rightarrow}{L}}_{o} \right)}} \\{= {{\overset{\rightarrow}{\Gamma}}^{- 1}\left( {{s\;{\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{I} \right)}} - {\overset{\rightarrow}{\Gamma}\left( \overset{\rightarrow}{W} \right)}} \right)}} \\{= \begin{pmatrix}{\Gamma^{- 1}\left( {{s\;{\Gamma\left( \max \right)}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s\;{\Gamma({mid})}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s\;{\Gamma\left( \min \right)}} - {\Gamma(W)}} \right)}\end{pmatrix}} \\{= {\begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 36}\end{matrix}$

For the variable scaling area VS, the luminance value of the white imagesignal is defined as the minimum of the components of the magnifiedvectors {right arrow over (L)}_(i), i.e., Γ(W)=s₂Γ(min) when the grayvector {right arrow over (O)} of the output three-color image signals isdetermined.

$\begin{matrix}\begin{matrix}{\begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix} = \begin{pmatrix}{\Gamma^{- 1}\left( {{s_{2}\;{\Gamma\left( \max \right)}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s_{2}\;{\Gamma({mid})}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{{s\;}_{2}{\Gamma\left( \min \right)}} - {\Gamma(W)}} \right)}\end{pmatrix}} \\{= \begin{pmatrix}{\Gamma^{- 1}\left( {{s_{2}\;{\Gamma\left( \max \right)}} - {s_{2}{\Gamma\left( \min \right)}}} \right)} \\{\Gamma^{- 1}\left( {{s_{2}\;{\Gamma({mid})}} - {s_{2}{\Gamma\left( \min \right)}}} \right)} \\{\Gamma^{- 1}\left( {{{s\;}_{2}{\Gamma\left( \min \right)}} - {s_{2}{\Gamma\left( \min \right)}}} \right)}\end{pmatrix}} \\{= {\begin{pmatrix}{\Gamma^{- 1}\left( {s_{2}\left\lbrack {{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\{\Gamma^{- 1}\left( {s_{2}\;\left\lbrack {{\Gamma({mid})} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\0\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 37}\end{matrix}$

Relation 37 is substituted with Relation 7 and using Relation 3,

$\begin{matrix}\begin{matrix}{\begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix} = \begin{pmatrix}{\Gamma^{- 1}\left( {\frac{\Gamma\left( \max \right)}{\begin{matrix}{{\Gamma\left( \max \right)} -} \\{\Gamma\left( \min \right)}\end{matrix}}\left\lbrack {{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\{\Gamma^{- 1}\left( {\frac{\Gamma\left( \max \right)}{\begin{matrix}{{\Gamma\left( \max \right)} -} \\{\Gamma\left( \min \right)}\end{matrix}}\left\lbrack {{\Gamma({mid})} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\0\end{pmatrix}} \\{= \begin{pmatrix}{\Gamma^{- 1}\left( {\Gamma\left( \max \right)} \right)} \\{{\Gamma^{- 1}\left( {\Gamma\left( \max \right)} \right)}{\Gamma^{- 1}\left( \frac{{\Gamma({mid})} - {\Gamma\left( \min \right)}}{{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right)}} \\0\end{pmatrix}} \\{= {\begin{pmatrix}\max \\{\max\;{\Gamma^{- 1}\left( \frac{{\Gamma({mid})} - {\Gamma\left( \min \right)}}{{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right)}} \\0\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 38}\end{matrix}$

The gray value W of the white signal is given by:

$\begin{matrix}\begin{matrix}{W = {\Gamma^{- 1}\left( {s_{2}{\Gamma\left( \min \right)}} \right)}} \\{= {\Gamma^{- 1}\left( \frac{{\Gamma\left( \max \right)}{\Gamma\left( \min \right)}}{{\Gamma\left( \max \right)} - {I^{\prime}\left( \min \right)}} \right)}} \\{= {\frac{\max \cdot \min}{\Gamma^{- 1}\left( {{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right)}.}}\end{matrix} & {{Relation}\mspace{14mu} 39}\end{matrix}$

For the fixed scaling area FS, if Relation 18 is not satisfied, theluminance value of the white image signal is defined as the minimum ofthe components of the magnified vector s{right arrow over (L)}_(i),i.e., Γ(W)=s₁Γ(min) when the gray vector {right arrow over (O)} of theoutput three-color image signals is determined.

$\begin{matrix}\begin{matrix}{\begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix} = \begin{pmatrix}{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma\left( \max \right)}} - {s_{1}{\Gamma\left( \min \right)}}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma({mid})}} - {s_{1}{\Gamma\left( \min \right)}}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma\left( \min \right)}} - {s_{1}{\Gamma\left( \min \right)}}} \right)}\end{pmatrix}} \\{= {\begin{pmatrix}{\Gamma^{- 1}\left( {s_{1}\left\lbrack \;{{\Gamma\left( \max \right)} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\{\Gamma^{- 1}\left( {s_{1}\;\left\lbrack {{\Gamma({mid})} - {\Gamma\left( \min \right)}} \right\rbrack} \right)} \\0\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 40}\end{matrix}$

The gray value W of the white signal is given by:W=Γ ⁻¹(s ₁(Γ(min)).  Relation 41

For the fixed scaling area FS, if Relation 18, i.e., s₁Γ(min)>W_(max) issatisfied, the luminance value of the white image signal is defined asthe highest value, i.e., Γ(W)=Γ(W_(max)) when the gray vector {rightarrow over (O)} of the output three-color image signals is determined.Accordingly,

$\begin{matrix}{\begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix} = {\begin{pmatrix}{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma\left( \max \right)}} - {\Gamma\left( W_{\max} \right)}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma({mid})}} - {\Gamma\left( W_{\max} \right)}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}\;{\Gamma\left( \min \right)}} - {\Gamma\left( W_{\max} \right)}} \right)}\end{pmatrix}.}} & {{Relation}\mspace{14mu} 42}\end{matrix}$

The gray value W of the white image signal is given by:W=Γ ⁻¹(Γ(max))=W _(max).  Relation 43

Now, a method of converting the three-color image signals into thefour-color image signals based on the rule described with reference toFIGS. 12A and 12B according to another embodiment of the presentinvention will be described in detail with reference to FIG. 14.

FIG. 14 is a flow chart illustrating a method of converting thethree-color image signals into the four-color image signals according toanother embodiment of the present invention.

When a set of three-color image signals having gray values R_(i), G_(i),and B_(i) (S31), the gray values R_(i), G_(i), and B_(i) are arranged inorder and assigned with order indices (S32). For example,If R_(i)≧G_(i)≧B_(i), Max=R_(i), Mid=G_(i), Min=B_(i), and OrderIndex=1;If R_(i)≧B_(i)≧G_(i), Max=R_(i), Mid=B_(i), Min=G_(i), and OrderIndex=2;If G_(i)≧B_(i)≧R_(i), Max=G_(i), Mid=B_(i), Min=R_(i), and OrderIndex=3;If G_(i)≧R_(i)≧B_(i), Max=G_(i), Mid=R_(i), Min=B_(i), and OrderIndex=4;If B_(i)≧R_(i)≧G_(i), Max=B_(i), Mid=R_(i), Min=G_(i), and OrderIndex=5; andIf B_(i)≧G_(i)≧R_(i), Max=B_(i), Mid=G_(i), Min=R_(i), and OrderIndex=6.  Relation 44

Then, the values Max, Mid, and Min are gamma converted (S33).

Next, it is determined using the values Γ(Max) and Γ(Min) obtained bygamma conversion which area the three-color image signals belong toamong the variable scaling area VS and the fixed scaling area FS (S34).That is, it is determined whether Γ(Max)>[s₁/(s₁−1)]Γ(Min). WhenΓ(Max)>[s₁/(s₁−1)]Γ(Min), the three-color image signals belong to thevariable scaling area VS and it goes to the step S35. If the relationΓ(Max)>[s₁/(s₁−1)]Γ(Min) is not satisfied, it goes to the step S36.

When the input image signals belong to the variable scaling area VS, thegray values Max′, Mid′, Min′, and W of the output four-color imagesignals are determined using Relations 38 and 39 (S35).

That is,Max′=Max;Mid′=Max Γ⁻¹{[Γ(mid)−Γ(min)]/[Γ(max)−Γ(min)]};Min′=0; andW=Max Min/Γ⁻¹[Γ(max)−Γ(min)].  Relation 45

When the input image signals belong to the fixed scaling area FS, it isdetermined whether s₁Γ(Min)>Γ(GV_(max)) (S36). Here, GV_(max) is thehighest gray as described above. This is to determine whether the grayvalue of the white image signal is higher than the highest gray.

If the above-described relation is not satisfied, the gray values Max′,Mid′, Min′, and W are determined by using Relations 40 and 41 (S37).Max′=Γ⁻ [s ₁Γ(max)−s ₁Γ(min)];Mid′=Γ⁻ [s ₁Γ(max)−s ₁Γ(min)];Min′=0; andW=Γ ⁻¹ [s ₁Γ(min)].  Relation 46

When s₁Γ(Min)>Γ(GV_(max)) is satisfied, the gray values Max′, Mid′,Min′, and W are determined by using Relations 42 and 43 (S38).Max′=Γ⁻¹ [s ₁Γ(max)−Γ(GV _(max))];Mid′=Γ⁻¹ [s ₁Γ(mid)−Γ(GV _(max))];Min′=Γ⁻¹ [s ₁Γ(min)−Γ(GV _(max))]; andW=GV_(max).  Relation 47

The Order Index conserves the order of the gray values of the inputsignals as follows:If Order Index=1, R_(o)=Max′, G_(o)=Mid′, and B_(o)=Min′;If Order Index=2, R_(o)=Max′, G_(o)=Min′, and B_(o)=Mid′;If Order Index=3, R_(o)=Min′, G_(o)=Max′, and B_(o)=Mid′;If Order Index=4, R_(o)=Mid′, G_(o)=Max′, and B_(o)=Min′;If Order Index=5, R_(o)=Mid′, G_(o)=Min′, and B_(o)=Max′; andIf Order Index=6, R_(o)=Min′, G_(o)=Mid′, and B_(o)=Max′.  Relation 48

Therefore, the gray values R_(o), G_(o), and B_(o) of red, green andblue output signals are determined by Relation 48 (S39).

Now, a method of determining the luminance value of the white signal inthe fixed scaling area FS according to another embodiment of the presentinvention is described in detail with reference to FIGS. 15 and 16.

FIG. 15 is a graph illustrating the luminance vector of the white signaland the luminance vector of the output three-color signals according toanother embodiment of the present invention.

Here, it is assumed in FIG. 15 that Y axis represents the image signalhaving the maximum luminance value and X axis represents another imagesignal.

This embodiment determines the maximum luminance values Γ(max′) of theoutput three-color signals to be equal to the maximum luminance valuesΓ(max) of the input three-color signals. That is,Γ(max′)=Γ(max).  Relation 49

At this time, the luminance vector {right arrow over (L)}_(w) of thewhite signal and the gray vector {right arrow over (O)} of the outputthree-color image signals can be obtained from Relations 35 and 36.

$\begin{matrix}{{\overset{\rightarrow}{L} = \begin{pmatrix}{\left( {s_{1} - 1} \right){\Gamma\left( \max \right)}} \\{\left( {s_{1} - 1} \right){\Gamma\left( \max \right)}} \\{\left( {s_{1} - 1} \right){\Gamma\left( \max \right)}}\end{pmatrix}};\mspace{14mu}{and}} & {{Relation}\mspace{14mu} 50} \\\begin{matrix}{\overset{\rightarrow}{O} = \begin{pmatrix}\max^{\prime} \\{mid}^{\prime} \\\min^{\prime}\end{pmatrix}} \\{= \begin{pmatrix}\max \\{\Gamma^{- 1}\left( {{s_{1}{\Gamma({mid})}} - {\Gamma(W)}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}{\Gamma\left( \min \right)}} - {\Gamma(W)}} \right)}\end{pmatrix}} \\{= \begin{pmatrix}\max \\{\Gamma^{- 1}\left( {{s_{1}{\Gamma({mid})}} - {\left( {s_{1} - 1} \right){\Gamma\left( \max \right)}}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}{\Gamma\left( \min \right)}} - {\left( {s_{1} - 1} \right){\Gamma\left( \max \right)}}} \right)}\end{pmatrix}} \\{= {\begin{pmatrix}\max \\{\Gamma^{- 1}\left( {{s_{1}\left\lbrack {{\Gamma({mid})} - {\Gamma\left( \max \right)}} \right\rbrack} + {\Gamma\left( \max \right)}} \right)} \\{\Gamma^{- 1}\left( {{s_{1}\left\lbrack {{\Gamma\left( \min \right)} - {\Gamma\left( \max \right)}} \right\rbrack} + {\Gamma\left( \max \right)}} \right)}\end{pmatrix}.}}\end{matrix} & {{Relation}\mspace{14mu} 51}\end{matrix}$

Now, a method of converting the three-color image signals into thefour-color image signals based on the rule described with reference toFIG. 15 according to an embodiment of the present invention is describedin detail with reference to FIG. 16.

FIG. 16 is a flow chart illustrating a method of converting thethree-color image signals into the four-color image signals according toanother embodiment of the present invention.

The conversion shown in FIG. 16 is almost the same as that shown in FIG.14. That is, when a set of red, green, and blue signals having grayvalues R_(i), G_(i), and B_(i) are received (S41), the gray valuesR_(i), G_(i), and B_(i) are arranged in order and assigned with orderindices (S42). For example, as in FIG. 14,If R_(i)≧G_(i)≧B_(i), Max=R_(i), Mid=G_(i), Min=B_(i), and Order Index1;If R_(i)≧B_(i)≧G_(i), Max=R_(i), Mid=B_(i), Min=G_(i), and Order Index2;If G_(i)≧B_(i)≧R_(i), Max=G_(i), Mid=B_(i), Min=R_(i), and Order Index3;If G_(i)≧R_(i)≧B_(i), Max=G_(i), Mid=R_(i), Min=B_(i), and Order Index4;If B_(i)≧R_(i)≧G_(i), Max=B_(i), Mid=R_(i), Min=G_(i), and Order Index5; andIf B_(i)≧G_(i)≧R_(i), Max=B_(i), Mid=G_(i), Min=R_(i), and Order Index6.  Relation 52

Then, the values Max, Mid, and Min are gamma converted (S43).

Next, it is determined, using the values Γ(Max) and Γ(Min) obtained bygamma conversion, which area the three-color image signals belong toamong the variable scaling area VS and the fixed scaling area FS (S44).That is, it is determined whether Γ(Max)>[s₁/(s₁−1)]Γ(Min). WhenΓ(Max)>[s₁/(s₁−1)]Γ(Min), the three-color image signals belong to thevariable scaling area VS and it goes to the step S45. If the relationΓ(Max)>[s₁/(s₁−1)]Γ(Min) is not satisfied, it goes to the step S46.

When the input image signals belong to the variable scaling area VS, thegray values Max′, Mid′, Min′, and W of the output four-color imagesignals are determined using Relations 38 and 39 like the method shownin FIG. 14 (S45).

That is,Max′=Max;Mid′=Max Γ⁻¹{[Γ(mid)−Γ(min)]/[Γ(max)−Γ(min)];Min′=0; andW=Max Min/Γ⁻¹[Γ(max)−Γ(min)].  Relation 53

When the input image signals belong to the fixed scaling area FS, thegray values Max′, Mid′, Min′, and W are determined by using Relation 51(S46).

That is,Max′=Max;Mid′=Γ⁻¹ {s ₁[Γ(mid)−Γ(max)]+Γ(max)};Min′=Γ⁻¹ {s ₁[Γ(min)−Γ(max)]+Γ(max)}; andW=Γ ⁻¹[(s ₁−1)Γ(max)].  Relation 54

When the scaling factor s₁ is two, for example, Relation 54 isrewritten,Max′=Max;Mid′=Γ⁻¹[2Γ(mid)+Γ(max)];Min′=Γ⁻¹[2Γ(min)+Γ(max)]; andW=Max.  Relation 55

The Order Index conserves the order of the gray values of the inputsignals as follows:If Order Index=1, R_(o)=Max′, G_(o)=Mid′, and B_(o)=Min′;If Order Index=2, R_(o)=Max′, G_(o)=Min′, and B_(o)=Mid′;If Order Index=3, R_(o)=Min′, G_(o)=Max′, and B_(o)=Mid′;If Order Index=4, R_(o)=Mid′, G_(o)=Max′, and B_(o)=Min′;If Order Index=5, R_(o)=Mid′, G_(o)=Min′, and B_(o)=Max′; andIf Order Index=6, R_(o)=Min′, G_(o)=Mid′, and B_(o)=Max′.  Relation 56

Therefore, the gray values R_(o), G_(o), and B_(o) of red, green andblue output signals are determined by Relation 56 (S47).

FIGS. 17A and 17B are graphs illustrating gamma curves that aregenerated by the methods shown in FIGS. 14 and 16. FIGS. 18A and 18B aregraphs illustrating a gamma curve of a white signal and a gamma curve ofthe output three-color signals, which are decomposed from the gammacurve shown in FIG. 17A. FIGS. 19A and 19B are graphs illustrating agamma curve of a white signal and a gamma curve of the outputthree-color signals, which are decomposed from the gamma curve shown inFIG. 17B.

FIGS. 17A-19B illustrate graphs showing gamma curves for achromaticcolor in a four-color LCD where L=α(GV)^(2.4) and the scaling factor s₁is equal to two. The horizontal axis represents the gray of the inputthree-color image signals and the vertical axis represents the lighttransmittance, i.e., the luminance. Here, the number of the grays isequal to 256 from zero to 255, which is available for 8-bit input imagesignals.

Although the gamma curve shown in FIG. 17A has an inflection point nearthe 192-th gray, the gamma curve shown in FIG. 17B has no inflectionpoint.

The difference is caused by the difference in the addition of the gammacurve of the white signal and the gamma curve of the output three-colorimage signals, i.e., the assignment of the luminance to the white signaland the output three-color signals.

In the method illustrated in FIG. 14, the white signal is assigned withthe maximum luminance by making the luminance vector {right arrow over(L)}_(w) of the white signal be determined by drawing a line until theline meets Y axis unless the gray is higher than the highest grayW_(max) as shown in FIGS. 12A and 12B.

Referring to FIGS. 17A and 17B, when the gray of the three-color inputimage signals is equal to the 192-th gray, for example, thetransmittance is equal to about 50%. The multiplication of the scalingfactor s₁ equal to two yields 100% transmittance, which corresponds tothe highest, 255-th gray of the white signal and to the zero-th gray ofthe output three-color image signals. When the gray of the inputthree-color signals is equal to 208 corresponding to the transmittanceof about 60%, the multiplication of the scaling factor s₁ yields thetransmittance of 120%. Then, the white signal is in charge of 100% andthe three-color image signals are in charge of remaining 20%. Then, thegray value of the white signal is equal to 255 as described above, andthe gray value of the output three-color image signals is equal to about128 corresponding to the transmittance of 20%.

To summarize, when the three-color input image signals are lower than192, only the gray of the white signal ranges from zero to 255. When thethree-color input image signals are in a range from 193 to 255, thewhite signal maintains its highest, 255 gray, while the outputthree-color image signals ranges from zero to 255.

In view of gamma curves shown in FIGS. 18A and 18B, the gamma curve W ofthe white signal has an exponential form from the zero-th gray to the192-th gray of the three-color input image signals and is saturated in50% transmittance from the 193-th gray to the 255-th gray. The gammacurve RGB of the output three-color signals exhibits an exponential formvarying as the input three-color input signal varies from the 193-thgray to the 255-th gray.

In the meantime, the gamma curve is an exponential function that has agradient increasing as the gray increases. Accordingly, the addition ofthe two exponential functions may yield an inflection point near the192-th gray and the 293-th gray, which correspond to the end point andthe start point of the two gamma curves, due to the difference in thegradient. The luminance difference near the inflection point is verysmall such that it is hard to distinguish the luminance differencebetween grays.

On the contrary, in the method shown in FIG. 16, a line segmentrepresenting the luminance vector {right arrow over (L)}_(w) of thewhite signal extends only to a point where it meets a rectangle having adiagonal OA as shown in FIG. 15. This means that both the two gammacurves operate in all the gray ranges.

When the scaling factor s₁ is equal to two, the white signal and thethree-color image signals are in charge of the luminance by a ratio of1:1 over all the grays as. For example, when the gray of the inputthree-color signals is equal to 192, the transmittance is equal to about50%, the multiplication of the scaling factor s1 yields 100%transmittance. Accordingly, the white signal is in charge of 50%transmittance, and the output three-color signals are also in charge of50% transmittance such that both the white signal and the outputthree-color signals have the 192-th gray. As a result, the gamma curve Wof the white signal is substantially equivalent to the gamma curve RGBof the output three-color signals as shown in FIGS. 19A and 19B, whichare added to each other to generate the gamma curve without aninflection point as shown in FIG. 17B. Although the gradient of thegamma curve may be varied depending on the scaling factor s₁, there maybe still no inflection point.

Accordingly, a gamma curve having no inflection point is generated,thereby obtaining distinct image quality.

According to the present invention, the removal of the discontinuity inthe gamma curve makes distinct images and the simplification of theconversion of the three-color image signals into the four-color imagesignals reduce the cost of the calculation chip and the calculationerrors such as quantization error.

In one embodiment, the gamma conversion and the reverse gamma conversionare performed by using a look-up table.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptsherein taught which may appear to those skilled in the present art willstill fall within the spirit and scope of the present invention, asdefined in the appended claims.

1. An apparatus for driving a display device including a plurality offour-color (a first color, a second color, a third color and a fourthcolor) pixels, the apparatus comprising: an input unit receiving inputthree-color (a first color, a second color and a third color) imagesignals; an image signal modifier converting the three-color imagesignals into output four-color image signals such that a maximum grayamong an input first color image signal, an input second color imagesignal, and an input third color image signal of the input three-colorimage signals is equal to a maximum gray among an output first colorimage signal, an output second color image signal, and an output thirdcolor image signal of the output four-color image signals; and an outputunit outputting the output four-color image signals, wherein the imagesignal modifier compares grays of the input three-color image signals,determines a maximum input gray, a middle input gray, and a minimuminput gray, and assigns order indices based thereon, gamma and reversegamma converts the maximum input gray, the middle input gray, and theminimum input gray to obtain a maximum output gray (Max′), a middleoutput gray (Mid′), a minimum output gray (Min′), and an output fourthcolor image gray (W) of the output four-color image signals, andgenerates the output four-color image signals based on the orderindices, wherein the maximum input gray (Max), the middle input gray(Mid), and the minimum input gray (Min) have relations with the maximumoutput gray (Max′), the middle output gray (Mid′), the minimum outputgray (Min′), and the output fourth color image gray (W) as follows:when Γ(Max)>[s ₁ /s ₁−1]Γ(Min),Max′=Max;Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];Min′=0; andW=Max Min/Γ⁻¹[Γ(Max)−Γ(Min)], andwhen Γ(Max)≦[s ₁ /s ₁−1]Γ(Min),Max′=Max;Mid′=Γ⁻¹{s₁[Γ(Mid)−Γ(Max)]+Γ(Max)]; andW=Γ ⁻¹[(s₁−1)Γ(Max)], wherein Γ is a gamma conversion function, Γ⁻¹ is areverse gamma conversion function, and s₁ is a scaling factor.
 2. Theapparatus of claim 1, wherein the first color, the second color, thethird color and the fourth color are selected among red, green, blue,white, cyan, magenta and yellow.
 3. The apparatus of claim 1, whereinthe image signal modifier further converts the three-color image signalsinto output four-color image signals such that another gray of theoutput first color image signal, second color image signal, and thirdcolor image signal is derived from the maximum gray and at least oneother gray of the input first color image signal, second color imagesignal, and third color image signal.
 4. The apparatus of claim 1,wherein the gamma function satisfies:Γ(xy)=Γ(x)Γ(y); andΓ⁻¹(pq)=Γ⁻¹(p)Γ⁻¹(q).
 5. The apparatus of claim 4, wherein the gammafunction is an exponential function.
 6. The apparatus of claim 5,wherein the power of the gamma function is equal to 2.4.
 7. Theapparatus of claim 1, wherein the scaling factor is equal to two.
 8. Theapparatus of claim 1, further comprising: a gray voltage generatorgenerating a plurality of gray voltages; and a data driver that selectsgray voltages among the plurality of gray voltages corresponding to theoutput four-color image signals and outputs the selected gray voltagesto the pixels as data voltages.
 9. The apparatus of claim 1, wherein theoutput fourth color image signal and output three-color image signals ofthe output four-color image signals are used together for substantiallyall grays of achromatic color.
 10. An apparatus for driving a displaydevice including a plurality of four color (a first color, a secondcolor, a third color and a fourth color) pixels, the apparatuscomprising: an input unit receiving input three-color (a first color, asecond color and a third color) image signals; an image signal modifierconverting the three-color image signals into output four-color imagesignals such that a gamma curve for achromatic color of the displaydevice has no inflection point below about a 94 percent transmittance ofthe gamma curve, wherein the gamma curve is a relationship between agray of one of the input three-color image signals and the transmittanceof a-corresponding one of the four-color image signals; and an outputunit outputting the output four-color image signals, wherein the imagesignal modifier compares grays of the input three-color image signals,determines a maximum input gray (Max), a middle input gray (Mid), and aminimum input gray (Min), assigns order indices based thereon, gamma andreverse gamma converts the maximum input gray, the middle input gray,and the minimum input gray to obtain a maximum output gray (Max′), amiddle output gray (Mid′), a minimum output gray (Min′), and an outputfourth color image gray (W) of the output four-color image signals, andgenerates the output four-color image signals based on the orderindices, wherein the maximum input gray (Max), the middle input gray(Mid), and the minimum input gray (Min) have relations with the maximumoutput gray (Max′), the middle output gray (Mid′), the minimum outputgray (Min′), and the output fourth color image gray (W) as follows:when Γ(Max)>[s ₁ /s ₁−1]Γ(Min),Max′=Max;Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];Min′=0; andW=Max Min/Γ⁻¹[Γ(Max)−Γ(Min)], andwhen Γ(Max)≦[s₁/s₁−1]Γ(Min),Max′=Max;Mid′=Γ⁻¹ {s ₁[Γ(Mid)−Γ(Max)]+Γ(Max)];Min′=Γ⁻¹ {s ₁[Γ(Min)−Γ(Max)]+Γ(Max)]; andW=Γ ⁻¹[(s ₁−1)Γ(Max)], wherein Γ is a gamma conversion function, Γ⁻¹ isa reverse gamma conversion function, and s₁ is a scaling factor.
 11. Theapparatus of claim 10, wherein the first color, the second color, thethird color and the fourth color are selected among red, green, blue,white, cyan, magenta and yellow.
 12. The apparatus of claim 10, whereinthe gamma function is an exponential function.
 13. The apparatus ofclaim 12, wherein the scaling factor is equal to two.
 14. The apparatusof claim 10, wherein a gray among an input R signal, an input G signal,and an input B signal of the input three-color image signals is equal toa maximum gray among an output R signal, an output G signal, and anoutput B signal of the output four-color image signals.
 15. Theapparatus of claim 10, wherein an output fourth color image signal andoutput three-color image signals of the output four-color image signalsare used together for substantially all grays of achromatic color.
 16. Amethod for driving a display device including a plurality of four color(a first color, a second color, a third color and a fourth color)pixels, the method comprising: assigning order indices after comparinggrays of the input three-color (a first color, a second color and athird color) image signals and determining a maximum input gray (Max), amiddle input gray (Mid), and a minimum input gray (Min); gammaconverting (Γ) and reverse gamma converting (Γ⁻¹) the maximum inputgray, the middle input gray, and the minimum input gray; obtaining amaximum output gray (Max′), a middle output gray (Mid′), a minimumoutput gray (Mid′), and an output fourth color image gray (W), fromrelationsMax′=Max;Mid′=Max Γ⁻¹[Γ(Mid)−Γ(Min)]/Γ(Max)−Γ(Min)],Min′=0, andW=Max Min Γ⁻¹[Γ(Max)−Γ(Min)],when Γ(Max)>[s ₁ /s ₁−1]Γ(Min); obtaining a maximum output gray (Max′),a middle output gray (Mid′), a minimum output gray (Mid′), and an outputfourth color image gray (W), from relationsMax′=Max;Mid′=Γ⁻¹ {s ₁[Γ(Mid)−Γ(Max)]+Γ(Max)],Min′=Γ⁻¹ {s ₁[Γ(Min)−Γ(Max)]+Γ(Max)], andW=Γ ⁻¹[(s ₁−1)Γ(Max)],when Γ(Max)≦[s ₁ /s ₁−1]Γ(Min); and generating four-color image signalshaving the maximum output gray, the middle output gray, the minimumoutput gray, and the output fourth color image gray according to theorder given by the order indices, wherein s₁ is a scaling factor. 17.The method of claim 16, wherein the first color, the second color, thethird color and the fourth color are selected among red, green, blue,white, cyan, magenta and yellow.
 18. The method of claim 16, wherein thegamma conversion and the reverse gamma conversion are performed by usinga look-up table.
 19. The method of claim 16, further comprising:generating a plurality of gray voltages; selecting data voltages amongthe plurality of gray voltages corresponding to the four-color imagesignals; and applying the data voltages to the pixels.